Solitary confinement is a form of imprisonment in which an incarcerated person lives in a single cell with little or no contact with other people. It is also known as isolation, SHU (special housing units), administrative segregation, supermax prisons, the hole, MCU (management control units), CMU (communications management units), STGMU (security threat group management units), voluntary or involuntary protective custody, special needs units, or permanent lockdown. The conditions of solitary confinement vary from state to state and among correctional facilities, but systematic policies and conditions include:

• Confinement behind a solid steel door for 22 to 24 hours a day
• Severely limited contact with other human beings
• Limited access to educational or vocational programs
• Limited access to healthcare services
• Limited access to natural light and fresh air
• Limited access to reading materials and other forms of entertainment

Solitary confinement is used on incarcerated individuals when they are considered a danger to themselves or others, or when they are suspected of organizing or being engaged in illegal activities within the facility. It is also used on individuals who are at high risk of being harmed by others, for example because they are transgender, have served as a witness to a crime, or have been convicted of crimes such as child molestation or abuse. This latter form of isolation is known as protective custody, and can be either voluntary or involuntary. Solitary confinement is also commonly used as punishment for those who have violated prison rules or committed other disciplinary infractions. The practice is the norm in super-maximum security (supermax) prisons, where individuals who are deemed dangerous or high risk are held.

Research has shown that solitary confinement has profound negative psychological, physical, and neurological effects on those who experience it, often lasting well beyond one’s time in solitary. The United Nations has considered solitary confinement torture when used for longer than 15 consecutive days. Despite this, the practice is widespread in jails, prisons, ICE detention centers, and juvenile facilities.